A difficulty regarding crankcase-scavenged engines is to provide a homogeneous air-fuel mixture to the combustion chamber. This can be achieved by so called long scavenging ducts, which however tends to make the crankcase complicated and bulky. For two-stroke engines provided with additional air to the scavenging ducts it is important to keep the air in the-scavenging ducts separated from the air-fuel mixture, in order to as far as possible prevent the air-fuel mixture from the scavenging ducts to disappear out through the exhaust port. This separation, also called stratification, is promoted by making the scavenging ducts long and narrow, thus preventing, or at least reducing, mixing of different scavenging gases.
For an engine body of the above-mentioned kind, where a cylinder is connected to the crankcase in a parting plane essentially perpendicular towards the cylinder bore, usually with a sealing intermediate layer, such as a gasket, the cylinder will end entirely above the crankshaft bearing and is therefore also called “short” cylinder. In order to prolong the scavenging ducts in such engine bodies it is known to let the scavenging ducts turn approximately 90 degrees in level with the contact surface and go round outside the cylinder wall in opposite directions, in order to meet in a mutual connecting duct leading into the crankcase. Since the scavenging ducts are located on essentially diametrically opposed sides of the cylinder each scavenging duct can in this way utilize an extra length corresponding to approximately a quarter of the circumference of the cylinder.
Anyhow, since this cannot always guarantee a satisfactory length of the scavenging ducts it occurs that said connecting duct extends down into the material of the crankcase walls and debouches into the bottom part of the inner space of the crankcase. This however results in a clumsy and bulky crankcase.
The scavenging ducts can be open and are in that case composed of grooves in the cylinder wall, which together with the piston form the scavenging ducts. However, in engines provided with additional air to the scavenging ducts, as well as in conventional, high-performance engines, the scavenging ducts are closed, i.e. they are separated from the cylinder bore by means of an intermediate wall. Usually closed scavenging ducts are vaulted out from the cylinder body for providing the scavenging gases a desired direction into and out from the cylinder bore. This design will lead to difficulties at die-casting of the cylinder body since the direction of the scavenging ducts will vary. Instead chill casting of the cylinder is more common, which is more expensive and more time-consuming.